Filter circuit



March 4, 1941. M. G. cRos sY 2,233,773

FILTER CIRCUIT Original Filed Jan. 22, 1938 3 Sheets-Sheet 1 vvvvvvvFILTER OUTPUT FILTER ourpur INVENTOR MURRAY G. ROSBY K ATTO R N EY March4, 1941. M, G. CROSBY 2,233,773

FILTER CIRCUIT Original Filed Jan. 22, 1938 3 Sheets-Sheet 2 v mmmMURRAY 6. R053) 1%! m ATTORNEY M. G. CROSBY FILTER CIRCUIT March 4, 1941.

3 Sheets- Sheet 3 Original Filed Jan, 22, 1958 FREQUENCY FREQUENCYFREQUENCY Patented Mar. 4, 1941 rarest or ies FILTER CIRCUIT Murray G.Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Original application January 22, 1938, SerialNo.

4 Claims.

This application which is a division of my United States applicationSerial No. 186,273 filed January 22, 1938, which issued as U. S. Patent2,204,574 June 18, 1940, concerns wave filters of the crystal type. Thewave filters of the present invention are of wide use in the radio artand involve the use of a piezo-electric crystal in a three-electrodeholder connected in circuits arranged to be excited by wave energy of asingle frequency or band of frequencies and to pass said wave energy toa plurality of output electrodes in modified form. The output electrodesmay comprise two pairs an electrode of each pair in some cases beingcommon. In describing my invention reference will be made to theattached drawings wherein,

Figures 1, 2 and 3 each show a different modification of my filtercircuits. Each embodiment o utilizes a piezo-electric crystal in aholder having three electrodes with means for adding to or taking fromthe reactance between the pairs of electrodes of the holder;

Figures 4a, 4b, 4c and 4 illustrate the characteristics of the filtercircuits illustrated in the circuits when arranged and adjusted inaccordance with my invention; while Figures 5a and 5b illustrate theband-pass characteristics of the filters when adjusted in a 30particular manner described more in detail later.

Wave energy is fed to the primary winding of tuned transformer I00. Thistransformer is properly clamped by resistances 99 and 90 and adjusted bycondensers 91 and 96 to form a band- 35 pass filter. The crystal I05 isheld in a crystal holder having a single input terminal IM and twooutput terminals I02- and I03. The input terminal is connected to thehigh potential side of the tuned transformer I00 and the two output 40terminals are connected to the plates II 4 and H6 of diodes I09 and H0.Tuned circuits I04, I06, and I01, I08 are connected between the commoninput terminal and the two output terminals. The resonant frequencies ofthese tuned 45 circuits are adjusted to be a little above and a littlebelow the resonant frequency of the crystal I05. This slight off-tuningproduces an inductive reactance in one case and a capacitive reactancein the other case. Hence the effect will be the 50 same as though acapacity were across one of the sets of crystal terminals and aninductance across the other set. This arrangement causes the formationof two crystal filters having overand under-neutralized characteristicsrespec- 55 tively. Thus, tuned circuit I04, I06 might be Divided andthis application April 20, 1940, Serial No. 330,656

tuned capacitive to present an under-neutralized characteristic such asshown at Figure 411 between transformer I00 and diode I09, and tunedcircuit I01, I08 might be tuned inductive to present an over-neutralizedfilter characteristic such 5 as shown at Figure 40 between transformerI00 and diode I I0. In my parent case these characteristics were used toconvert phase modulation on wave energy into amplitude modulation onwave energy.

In the circuit of Figure 2 the same principles are applied except thatsingle reactances I06 and Ill? take the place of the tuned circuits ofthe prior figure connected across the crystal holder terminals anddiode-driver amplifier tubes I09 and H0 having their grids H3, H5 andcathodes connected to the crystal output electrodes are excited by thefiltered energy and are used to feed the two filtered energies to thediodes I09 and H0. Tuned band-pass transformers I28 and I30, havingtheir primaries shunted by damping resistances I and I21 couple theanodes and cathodes of the driver tubes I00 and III! to the diode platesH0 and H6.

In the circuit of Figure 3 a discriminator is shown which utilizes ashunt connected crystal in the manner of the discriminators described inmy application #167,344 filed October 5, 1937, now Patent No. 2,156,375dated May 2, 1939. In: termediate frequency energy is fed to tunedtransformer I00 and through isolating resistors 200 and 20I to the twoterminals of the crystal holder I02 and I03. The common terminal IOI ofthe crystal holder is connected to the low side of the tunedtransformer. Reactances XI and X2 would be either an inductance and acapacity or two tuned circuits. These reactances are adjusted so thatone forms a capacitive action across the crystal holder and the other aninductive action. The output of the two filters is taken from terminalsI02 and I03 and fed to the grids H3 and H5 of diode-driver tubes I09 and0' such as are used in Figure 2. The other features of this circuit arethe same as those of Figure 1 and Figure 2.

The characteristic of the energy present on the input electrodes of I09of Figure 1 and in, say, transformer I28 of Figures 2 and 3, is as shownin Figure 4a being derived from a filter of the under-neutralized typesuch as is obtained by under-neutralizing the holder electrodes IIII andI02 by means of I06 of Figure 2 or the reactance at XI of Figure 3 orI04 and I06 of Figure 1. This is obtained by adjusting the said elementsof the respective filters so that the reactance across the crystalholder is capacitive for the case of the series connected crystals ofFigures 1 and 2 and inductive for the case of the shunt connectedcrystals of Figure 3. The characteristic of the energy, say intransformer I of Figures 2 and 3, or on the electrodes of diode III! ofFigure 1 is as shown in Figure 4e, this energy being passed by filterswhich are over-neutralized by adjusting the elements I01 and I08 ofFigure 1 or I01 of Figure 2 so that the crystal holder has an inductivereactance at substantially the carrier frequency, and adjusting thereactance X2 of Figure 3 so that the crystal holder has a capacitivereactance. In the case of the shunt connected crystal, theunder-neutralized characteristic is given by Figure 4e and its reactancecharacteristic is given by Figure 4b. That is, in the case of the shuntconnected filter the low impedance point on the reactance curve (Wherethe line crosses the X axis) coincides with the rejection point on theoutput characteristic. Also, the high impedance point on the reactancecurve coincides with the carrier frequency. This is opposite to the caseof the series connected filter in which the under-neutralizedcharacteristic is given by Figure 4a, its reactance curve is given by4b, the zero reactance curve coincides with the carrier frequency, andthe maximum impedance point coincides with the rejection point. r

The reason for this diiference between the shunt and series connectedfilters is the fact that with the series connection th crystal allowsmore or less energy to fiow according to its impedance, while with theshunt connection, since the drop across the crystal is utilized, theoutput voltage is proportional to the crystal impedance.

In the case of Figure 4b a negative reactance is present on both sidesof the carrier frequency, that is, the circuit would be capacitive onboth sides of the carrier except for the very short interval near thecarrier frequency.

I have found that by adjusting the crystal filters so that one has avery small amount of negative reactance across the holder and the othera very small amount of positive reactance, a single sideband efiect isobtained as shown in Figures'5a and 5b. With the crystals almostneutralized in this way the output on the side of the carrier frequencywhich does not have the rejection point is higher than the output on theside having the rejection point. Consequently, one of the filters exaltsthe carrier and selects the lower sideband and the other exalts thecarrier and selects the upper sideband.

For more detailed description of the characteristics and adjustments ofmy filter circuit and for the various uses to which the same may be put,reference is made to the parent applicationSerial No. 186,273 filedJanuary 22, 1938 (Patent 2,204,574)

What is claimed is:

1. In a Wave filtering system, an impedance on which wave energy to befiltered may be impressed, an output circuit, a piezo-electric crystalhaving three electrodes, meansconnecting a pair of said electrodes andsaid impedance in a first filter circuit coupled to said output circuit,means connecting a second pair of said electrodes and said impedance ina second filter circuit coupled to said output circuit and. reactancesconnected in shunt to each of said pair of crystal electrodes, tosupplement the reactance between both pairs of electrodes.

2. In a wave filtering system, an impedance on which wave energy to befiltered may be impressed, an output circuit, a piezo-electric crystalhaving three electrodes, means connecting a pair of said electrodes andsaid impedance in series in a first filter circuit coupled to saidoutput circuit, means connecting a second pair of said electrodes andsaid impedance in series in a second filter circuit coupled to saidoutput circuit and reactances connected in shunt to each of said pair ofcrystal electrodes, to supplement the reactance between both pairs ofelectrodes.

3. In a filter circuit, an impedance on which wave energy to be filteredmay be impressed, an output circuit, a piezo-electric crystal havingthree electrodes, means connecting a pair of said electrodes and saidimpedance in a first filter circuit coupled to said output circuit,means connecting a second pair of said electrodes and said impedance ina second filter circuit coupled to said output circuit, a reactanceconnected in shunt to one of said pair of electrodes, of a nature suchthat the reactive effect of the crystal electrodes is inductive, and areactance connected in shunt to the other pair of said electrodes of anature such that the reactive efiect of the crystal electrodes iscapacitive.

4. In a filter circuit, an impedance on'which wave energy to be filteredmay be impressed, an output circuit, a piezo-electric crystal havingthree electrodes, means connecting a pair of said electrodes and saidimpedance in a first filter circuit coupled in shunt to said outputcircuit,

means connecting a second pair of said electrodes and said impedance ina second filter circuit coupled in shunt to said output circuit, areactance connected in shunt to one of said pair of electrodes, of anature such that the reactive effect is inductive, and a reactanceconnected in shunt to the other pair of said electrodes of a nature suchthat the reactive effect is capacitive.

MURRAY G. CROSBY.

